Answer by asmgx for Reading settings from app.config or web.config in .NET

I found the answer in this link https://stackoverflow.com/a/1836938/1492229

It's not only necessary to use the namespaceSystem.Configuration. You have also to add the reference to the assemblySystem.Configuration.dll , by

1. Right-click on the References / Dependencies
2. Choose Add Reference
3. Find and add System.Configuration.

This will work for sure.Also for the NameValueCollection you have to write:

using System.Collections.Specialized;